1. Field of the Invention
The present invention relates to a steam cooled type gas turbine in which steam is used as a gas turbine cooling medium.
2. Description of the Prior Art
As one example of a prior art gas turbine cooling structure, cooling of an air cooled type stationary blade will be outlined with reference to FIG. 4.
In a gas turbine having a system to cool a high temperature portion using air as a cooling medium, cooling air is led from a stationary blade outer shroud side as shown by arrows a. A portion of the air so led flows in the blade for cooling of an inner surface thereof and then flows out of holes provided in the blade to flow along the blade surface for cooling thereof as shown by arrows b, and is discharged in a combustion gas passage as shown by arrows e.
A remaining portion of the air does not flow out of the holes provided in the blade but is led into a stationary blade inner shroud for cooling of the inner shroud and a blade lower portion and then flows as shown by arrow c to be discharged into the combustion gas passage.
In the above, in order to prevent the air so led as shown by arrows a from leaking directly into the combustion gas passage before it performs its original duty to cool the blade portion and the blade lower portion, there are provided seals d at contact surfaces between the blade and the outer shroud and between the blade and the inner shroud.
As mentioned above, in the air cooled type gas turbine, after the air as cooling medium has cooled the structural body of the stationary blade, shrouds, etc., it is discharged into the combustion gas passage without being recovered, hence there is less thermal efficiency in this system.
Thus, as part of a recent tendency to obtain a better system in thermal efficiency, a steam cooled type gas turbine has been disclosed as another prior art example in which steam is used as the cooling medium in place of air.
As to this prior art example, although illustration is omitted, steam as a cooling medium is supplied into portions to be cooled of the gas turbine, including the structural body of the stationary blade, shrouds, etc., and the steam which has been heated through cooling of the cooled portions is recovered so that the heat of the steam is utilized effectively and enhancement of the thermal efficiency can be attained.
In this steam cooled system, however, in order to ensure recovery of the heat obtained in the cooled portions, there is needed an accurate and strong construction of cooling passages in which the steam does not leak, not only into the combustion gas passage, but also along the way of steam flow passages.
Hence, in the steam cooled system, construction thereof is preferably such that steam supply ports and recovery ports are lessened to the extent possible, and stationary blades of a first stage, second stage and subsequent stages are supplied with the cooling steam for cooling thereof through a single steam passage, and then the steam is recovered.